Catalyst components for the polymerization of olefins, such as ethylene and propylene, are widely known in the art and include Ziegler-Natta (Z-N) type catalysts. The first catalyst of this type widely used in the industry was based on the use of solid TiCl3 obtained by the reduction of TiCl4 with aluminum alkyls. The activity and stereospecificity of early Z-N catalysts were not ideal so the polymer had to undergo a de-ashing treatment to remove the catalyst residues, and a washing step for removing the atactic polymer (polypropylene) produced. Ziegler-Natta catalysts used industrially often comprise a solid catalyst component, constituted by a magnesium dihalide on which a titanium compound and optionally an internal electron donor compound, used in combination with an Al-alkyl compound, are supported.
The use of magnesium chloride based supports led to increases in the catalyst activity and to the development of various techniques for their production. For instance, in ethylene polymerization, catalyst activity is important. In U.S. Pat. No. 4,330,646, an organomagnesium hydrocarbon soluble component is reacted with an halide of boron, silicon, germanium, tin, phosphorous, antimony bismuth or zinc to produce magnesium chloride, which is then subject to a titanation reaction. There is no evidence that the metal of the halogenating compound in this process remains fixed on the catalyst support at the end of the preparation process. However, in terms of catalyst activity, the use of SbCl3 and Sb(C2H5)3Cl2 as a halogenating agent (please see Examples 4 and 25) may generate a less active catalyst in an ethylene polymerization carried out with the catalyst lacking an internal donor. It would therefore be beneficial to develop a catalyst with increased catalyst activity in ethylene polymerization.
When Z-N catalysts are used for propylene polymerization, they may contain an internal donor. Moreover, they may be used together with an external donor (for example, an alkoxysilane) which may produce higher isotacticity. Internal donors may include the esters of phthalic acid, such as diisobutylphthalate. Phthalates are often used as internal donors in combination with alkylalkoxysilanes as external donors. This catalyst system is capable of good performance in terms of activity, and can produce propylene polymers with high isotacticity and xylene insolubility. However, increasing the intrinsic capability of the solid catalyst components, including those based on donors different from phthalates, to produce stereoregular polymers would be beneficial in propylene polymerization. In fact, an intrinsically more stereospecific catalyst component would allow the use of a lower amount of stereoregulating internal and/or external donor to reach the target of polymer xylene insolubility and this, in turn, would be translated into the possibility of obtaining higher plant productivity.
Based on this, it would be very convenient to find a way of improving the stereospecificity of a solid catalyst component, and it would be convenient if this method had wide applicability.
U.S. Pat. No. 4,237,254 discloses catalyst preparation methods in which a magnesium hydroxychloride support is converted into a magnesium chloride based catalyst by reacting it with halogenating agents such as benzoylchloride. The support is then co-milled with a benzoate as internal donor and treated with TiCl4. Further treatment of the catalyst with an additional halogenating agent such as BiCl3 may also be useful. However, the use of the additional chlorinating agents helps in terms of stereospecificity in connection with the use of benzoates as internal donors was not investigated. The applicant demonstrated, however, that the use of BiCl3 in the catalyst along with benzoates as internal donors offered no improvement in activity/stereospecificity.
U.S. Pat. App. Pub. No. 2013/0244863 describes a catalyst preparation in which a Mg complex containing an acid salt of a Group IB-VIIIB element (CAS version of the periodic table of elements) and an internal donor is contacted with a titanium compound to produce a catalyst component to be used in a catalyst system comprising an aluminum compound co-catalyst and a silicon compound as an external donor. Notwithstanding the vast number of compounds included in a potential acid salt of a Group IB-VIIIB element, only few salts were tested (Table 1), none of them belonging to Group VA of the Periodic Table of Elements. In addition, the results are contradictory in many cases. For instance, Table 2 (a catalyst containing phthalate as an internal donor) shows that the comparative catalyst not containing the Group IB-VIIIB salt has activity and stereospecificity higher than that of many (1-4, 6-11, 13-15 and 17-19) of the inventive examples disclosed therein. When a diether is used as an internal donor a substantial increase of stereospecificity is generally seen only with CuCl2 and PdCl2. Due to the fact that in Table 1 of the referenced application the performance of catalyst including ZnCl2 is very low, it seems that, in going from Group IB to Group IIB, the performances are deteriorating.
The applicants have surprisingly found that, when the disclosed catalyst contains a certain amount of bismuth (Bi) atoms, improvement in activity and/or stereospecificity is obtained in polymerizing olefins such as ethylene or propylene.